The effect of acidic solutions and growth conditions on the adsorptive properties of bacterial surfaces

Data must be collected over a wide pH range to accurately model the adsorption of protons and metal onto bacterial surfaces; however, alterations in the structural and chemical properties of bacterial surfaces resulting from exposure to acidic solutions may affect the mechanisms of cation binding. Binding properties of bacteria may also be affected by nutrient and oxygen levels present during their growth. We measured Cd, Co, and Pb adsorption onto bacteria by using (1) bacteria washed with acidic solutions (pH≈1.5), (2) non-acid-washed bacteria exposed to acidic parent solutions, and (3) non-acid-washed bacteria exposed to neutral parent solutions. The purpose was to determine the effect of acidic conditions on the adsorptive properties of Pseudomonas mendocina, Pseudomonas aeruginosa, Bacillus subtilis, and Bacillus cereus. We also measured Co adsorption onto bacteria (Pseudomonas fluorescens) grown under nutrient-rich and nutrient-limited conditions and onto the facultative bacterium Shewanella oneidensis MR-1 grown under aerobic and anaerobic conditions. Bacteria exposed to acidic solutions adsorbed more metals than bacteria not exposed to such solutions. We attribute the increase in adsorption to the irreversible displacement of structurally bound Mg and Ca by protons. After displacement, the protonated sites can participate in reversible metal adsorption reactions. Thermodynamic modeling suggests that concentrations of functional group sites on bacterial surfaces increase by as much as five times in response to acid washing, assuming that stability constants for the bacterial surface complexes remain the same. Although the sizes of the bacteria changed markedly in response to nutrient limits and oxygen content during growth, the mass-normalized extent of Co adsorption onto both P. fluorescens and S. oneidensis MR-1 was independent of growth conditions. We conclude that adsorption constants derived from experiments in which the bacteria are never exposed to acidic conditions probably provide the most accurate estimates of the extent of bacteria–metal adsorption in natural settings.